Determine RF input power

in designing down converter mixer, how to determine RF input power needed? Most of designing circuit they chose -30dB.
thank you

if u want to know the conversion gain or noise figure, you should use the small signal analysis. in small signal analysis no matter how large the RF signal u choose, it will not influence the circuit.
if u want to know the linearity of your circuit, u should sweep you RF signal from very small one to very large one.

For the mixer to work properly, the LO signal must be the largest signal in the mixer.

A typical rule of thumb is that the LO should be 30 dB higher than the RF. The actual level is determined by your linearity requirement, but 30 dB is good starting point.

pstuckey said:

For the mixer to work properly, the LO signal must be the largest signal in the mixer.

A typical rule of thumb is that the LO should be 30 dB higher than the RF. The actual level is determined by your linearity requirement, but 30 dB is good starting point.

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thank you. For example if rf -30dB and LO 0 dB, is it Ok? or find out the maximum LO power from voltage conversion vs LO signal power then only adjust RF power base on LO power?

THere are more parameters to consider.
First, your down-converter mixer will be a part of a receiver. To function, you need to know the type of signal modulation and transmission data rate. This requires a certain receiver bandwidth. The bandwidth determines the input noise power, the "N" in signal-to-noise ratio.
Second, to use a RF input signal in any receiver, its power must exceed that of noise. Common communication systems need the signal power, "S", to be 10-100x higher than the input noise, in other words, S/N ratio must be at least 10 to 20 dB for a system to operate.
Third, depending on mixer design, mixers must be "pumped" by LO input power, typically +10 dBm or more for balanced diode mixers. LO frequency must be set so as to create the output IF frequency as a difference (or sum in some cases) between the RF input and LO input frequency.